Microscopy of live cells plays an important role in biomedical sciences. These are usually cultivated in vessels such as microtiter plates or Petri dishes. The cells are located on the bottom of the vessel and are surrounded by a culture medium. They are subjected to microscopy with an inverted microscope; in this microscope the objective is located beneath the bottom of the sample vessel. The sample can be illuminated via incident light or transmitted light. For transmitted-light images a light is attached above the sample vessel. However, as biological cells contain only few absorptive constituents, bright-field transmitted-light images are typically contrasted only very weakly. With the help of various transmitted-light contrasting methods, such as phase contrast, DIC, inter alia, the small difference in refractive index of the individual cell constituents from one another and from the surrounding medium can be converted into an intensity difference which then provides a contrasted transmitted-light image, but from this frequently only little can be said about the functions or distributions of specific substances within the cell. Fluorescence microscopy solves this problem by specific substances (fluorophores) already available to the cells or which have been introduced into same being excited with the help of incident-light illumination, which fluorophores then in turn emit a signal which is captured by the objective and is transmitted to the camera or the oculars. Because, however, only those structures can be seen, the fluorophores of which are excited, information about the size and shape of the individual cells can become lost. Therefore, incident and transmitted light are frequently combined as they provide complementary information.
Living cells are not static objects but are permanently changing as they are alive. In particular, they can act as the subject of studies into the effects of environmental influences on living organisms. These influences also include the material composition of the nutrient medium. If this is changed, the cell reacts to it. The reaction times can lie in the range of minutes and hours, but also seconds. It may thus be of decisive importance that observation takes place immediately after or indeed during a change to the cell environment. The change to the environment is usually achieved by the pipetting the substance, the effect of which is to be investigated, into the nutrient solution. As the sample vessels are open at the top, the reagents are also introduced from the top. Thus a spatial conflict with the transmitted-light illumination is intentional.